Caption: Cylindrospermum algae. Light micrograph of filaments of Cylindrospermum sp., a unicellular blue-green alga or cyanobacterium that can absorb (fix) atmospheric nitrogen. The smaller algal cells use the pigment chlorophyll to absorb sunlight during the food-making process of photosynthesis. The algae can form larger cells known as heterocysts which specialise in fixing nitrogen, an essential nutrient for biological growth. The metabolism of the Cylindospermum algae is unusual and they do not appear to have enzymes for breaking down sugars (glucose) to produce energy. Magnification: x160 at 35mm size.